Organic light-emitting diodes (OLED) generate light by re-combination of electrons and holes, and emit light when a bias is applied between the anode and cathode such that an electrical current passes between them. The brightness of the light is related to the amount of the current. If there is no current, there will be no light emission, so OLED technology is a type of technology capable of absolute blacks and achieving almost “infinite” contrast ratio between pixels when used in display applications.
Several approaches are taught in the prior art for pixel thin film transistor (TFT) circuits to deliver current to an element of a display device, such as for example an organic light-emitting diode (OLED), through a drive transistor. In one example, an input signal, such as a high “SCAN” signal, is employed to switch transistors in the circuit to permit a data voltage, VDAT, to be stored at a storage capacitor during a programming phase. When the SCAN signal is low and a switch transistor isolates the circuits from the data voltage, the VDAT voltage is retained by the capacitor and this voltage is applied to a gate of a drive transistor. With the drive transistor having a threshold voltage VTH, the amount of current to the OLED is related to the voltage on the gate of the drive transistor by:
      I    OLED    =            β      2        ⁢                  (                              V            DAT                    -                      V            OLED                    -                      V            TH                          )            2      
TFT device characteristics, especially the TFT threshold voltage VTH, may vary, for example due to manufacturing processes and/or stress and aging of the TFT device during the operation. With the same VDAT voltage, the amount of current delivered by the TFT drive transistor could vary by a large amount due to such threshold voltage variations. Therefore, pixels in a display may not exhibit uniform brightness for a given VDAT value. In addition, OLED device characteristics may vary due to manufacturing processes, and/or stress and aging during the operation of the OLED. For example, the threshold voltage of the OLED for light emission may change. Conventional circuit configurations, therefore, often include elements that operate to compensate for at least some of these component variations to achieve an OLED display with more uniform brightness among sub-pixels.
Accordingly, there are various methods that have been employed to compensate for the drive TFT and OLED variations. Normally, such methods use a circuit configuration having several transistors. The size required by many of these circuit configurations may not be suitable for high resolution displays having high pixels per inch (ppi), in which each subpixel occupies only a small area. With a small area and limited number of transistors, external measurement and compensation may be used to measure the circuit performance and device properties, such as threshold voltage and mobility of the drive transistor and the OLED. An external circuit, therefore, may be used to generate a compensated data signal according to those measurements for appropriate compensation.
One approach for external compensation is described in U.S. Pat. No. 6,433,488 (Bu, issued Aug. 13, 2002). During measurements, the current to the OLED is diverted to an external current comparator and compared with a reference current. An output data voltage is used to adjust the current through the drive transistor. This approach is deficient, however, because the current is diverted from the OLED, and thus the OLED properties are not measured. The drive transistor may not work in the same condition as in the emission phase, and thus the compensation measurements may not be accurate.
Another approach for external compensation is described in U.S. Pat. No. 7,876,292 (Cho et al., issued Jan. 25, 2011). During the measurement stage, the current through the OLED is sensed and compared with a reference current and then the difference is converted to a data voltage. This data voltage is then applied to the gate of the drive transistor. This approach also is deficient because the anode and cathode of the OLED have to connect to the pixel circuit. During the manufacturing process, normally only one of the nodes, the anode or cathode, is accessible from the pixel circuit, and thus compensation during use may not be sufficiently accurate.
Another approach for external compensation is described in U.S. Pat. No. 9,336,717 (Chaji, issued May 10, 2016). A monitor line is used to monitor the current through the drive transistor while the OLED is turned off. The programming data voltage is adjusted according to the monitored current. This approach is deficient because the pixel circuits have five transistors, and thus the number of transistors is undesirably high for use with an external compensation scheme.
Another approach for external compensation is described in U.S. Pat. No. 9,997,106 (Chaji, issued Jun. 12, 2018). A monitor line is used to monitor the current through the drive transistor while the OLED is turned off. The programming data voltage is adjusted according to the monitored current. In the pixel circuits, the storage capacitor is connected between the source and gate of the drive transistor. This approach is deficient because the parasitic capacitance of the OLED could affect the charge distribution between the storage capacitor and the parasitic capacitors. The OLED mismatch could cause more variations in light emission than other methods.
It is desirable that the programming time for the pixel circuitry be as short as possible for optimal performance of the display system. A significant challenge with respect to conventional external compensation systems is minimizing the programming time while maintaining a compact pixel circuit arrangement.